The development of lactic acidemia (LA) in septic shock (SS) is associated with an ominous prognosis (1). Blood concentrations of lactate depend on the rates of its production and utilization by various organs (2). In an experimental canine model of SS, we previously showed that production of lactate by the splanchnic organs increased in this model, while hepatic uptake of lactate was impaired (3). The mechanism by which hepatic impairment may occur in SS is not clear. Uptake of lactate by the liver occurs by means of a membrane-associated, pH-dependent, bidirectional facilitative antiport transport system known as the monocarboxylate transporter (MCT) (4-6). MCT1 is highly expressed in the liver. In the condition in which there is increased production of lactate by means of the glycolytic pathway in the liver, lactate could exit the hepatocyte by means of MCT, while there would be a cotransport of hydroxyl (OH) anions into the cell. This would lead to excess hydrogen ions into the extracellular space and the clinical syndrome of LA (see FIG. 1A). On the other hand, hepatic uptake of lactate by MCT would produce a cotransport of OH ions into the extracellular space ions and a correction of LA. The extent to which MCT protein may be altered in SS has never been studied.
Once transported into the hepatocyte by MCT, lactate can be shuttled into the Cori-cycle for gluconeogenesis or can be converted to pyruvate to enter the mitochondria to be metabolized to CO2 and water (6) (see FIG. 1). Mitochondrial dysfunction has been postulated as an explanation for the development of LA in SS, although this has been not universally accepted (7-15). In mitochondria, electrons are transferred primarily by reduced nicotinamide adenine dinucleotide (NADH) to mitochondrial complexes I, III, and IV to generate a proton (H+) gradient (the proton motive force; pmf) across the mitochondrial inner membrane. The free energy accumulated as the pmf is then used to drive ATP synthesis through the F1FoATP synthase activity (Complex V) allowing protons to return to the mitochondrial matrix (12-13).
The variable results previously reported in the literature about the presence of mitochondrial dysfunction in SS may be a consequence of the different methodology and experimental protocols that were used in these studies. The recent development of the Seahorse XF24 extracellular flux analyzer that we used in the present study allows for precise measurements of mitochondrial oxygen consumption under various conditions and has not previously been used to investigate mitochondrial dysfunction in SS (17).